1. Field of the Invention
The present invention relates to a diglyceride-containing oil or fat composition which is stable against thermal oxidation or hydrolysis and which has excellent flavor and appearance, to fried foods produced by use of the composition, and to a method of producing the fried foods.
2. Discussion of the Background
Recently, research has progressed to unveil a correlation between the distribution of fat in the body and a variety of life-style related diseases. Specifically, accumulation of visceral fat, such as fat in the abdominal cavity or liver fat, has bean shown to be highly correlated with not only obesity but also diabetes, hyperlipemia, liver diseases, hypertension, and other conditions and diseases. Therefore, reduction of body fat is considered critical for the prophylaxis and treatment of these conditions or diseases.
Exercise and diet are important means of reducing body fat, but in many cases difficulty is encountered in proper diet control. Pharmaceutical therapy is another approach for reducing body fat, but can involve a concern in terms of safety.
Diglycerides, which are known to have an effect of reducing accumulation of body fat and preventing obesity, have recently become of interest, and diglyceride-containing edible oils have already been on the market (see Japanese Patent Application Laid-Open No. 10-176181). However, like triglycerides, diglycerides are susceptible to hydrolysis and oxidation when stored or heated for a long time. Fatty acid generated during hydrolysis lowers the smoke point of oil, to thereby permit issuance of odor during cooking and to also hamper efficient cooking work. Also, oxidation raises problems, such as deteriorative smell and coloring of the oil. Hitherto, no effective method has been known to improve the stability of triglycerides against hydrolysis. Meanwhile, techniques are known to prevent oxidation of triglycerides, including addition of a variety of anti-oxidation agents to frying oil or fat (hereinafter “oil or fat” may be referred to simply as oil, and the term “frying” is used to encompass stir-frying, deep-frying, and any other similar methods of cooking by use of oil) and addition of citric acid as a synergist for the anti-oxidation agent (“Theories and Practice of Fried Foods” published by Saiwai Shobo, 1976). In particular, citric acid or a salt thereof employed in the latter method is sparingly soluble to triglycerides, and improvement is desired with respect to its effect (Japanese Patent Application Laid-Open No. 49-86557).
In 1983, isopropyl citrate was approved as a food additive for its function of improving solubility of citric acid as a salt thereof to triglycerides. However, since isopropyl citrate is produced from isopropyl alcohol, the amount of isopropyl citrate which can be incorporated into an oil or fat is currently restricted to 100 ppm or less (Manual for Japanese Official Compendium of Food Additives, 1999).
In connection with oils and fats consumed under severe conditions by professionals engaged in culinary enterprises such as restaurants, fast food shops, and fried food manufacturing plants, there remains a strong need for improved stability of triglycerides and diglycerides against oxidation. In order to meet this need, in same cases, ascorbyl palmitate, tocopherol, as a similar compound capable of serving as an antioxidant is used in large amounts. However, addition of these antioxidants in large amounts increases acid value, which is associated with an increased amount of smoke and a change in oil color during cooking. Moreover, fried foods produced by use of an oil containing such an additive involve problems in terms of quality; e.g., degradation in flavor, color tone, and appearance, which is partly attributed to soap smell from fatty acid generated through decomposition of the oil.
There is proposed a method for preventing oxidation by use of catechin (WO 00/77133). Although catechin significantly improves stability of oil or fat against oxidation, its effect of preventing hydrolysis of oil or fat is not necessarily satisfactory.
It is also disclosed, a method for preventing disproportionation from diglycerides to triglycerides and monoglycerides during a deodorization process by use of citric acid (Japanese Patent Application Laid-Open No. 4-261497). However, the document has no indication that citric acid prevents hydrolysis of diglycerides containing oil.
In view of the foregoing, the present inventors have performed research on the stability of oils or fats containing diglyceride, which has the effect of preventing accumulation of body fat, against hydrolysis and thermal degradation caused by long-term storage or heating, and have discovered a peculiar phenomenon that organic carboxylic acids, which are sparingly soluble to triglycerides and thus are unsatisfactory in their effect to stabilize triglycerides, are abundantly soluble to diglycerides. They have also discovered that organic carboxylic acids exhibit a significant effect on the aforementioned stability; in particular, prevention of hydrolysis of a diglyceride-containing oil or fat. In short, the present inventors have found that when a certain amount of organic carboxylic acid is added to an oil or fat of high diglyceride content, there can be obtained an oil or fat composition for frying use which is stable against thermal oxidation or hydrolysis—which conventionally cannot be avoided after long-term storage or heating—and whose smoke point is resistant to lowering during the course of cooking, and that cooking by use of the oil composition provides fried foods of excellent flavor and appearance.